Currently, linear alternators used in conjunction with engines, motors, etc. (e.g., Stirling engines, etc.) are typically temperature limited to less than 250° C. operation due to insulation, adhesive, and permanent magnet operating temperature limits. In addition, state of practice linear alternators experience eddy current losses in their iron laminations as well as in their permanent magnets from stator back EMF. These eddy losses result in a total mechanical to electrical conversion system efficiency of less than 95% and usually in the 88%-90% performance range under realistic operating conditions. Moreover, permanent magnets often require adhesives that can out-gas and degrade over time.
Three commonly employed linear alternator architectures and their disadvantages include: moving iron (adds moving weight and hence requires additional costly reactive forces), moving magnet (which can damage somewhat fragile permanent magnets), or moving coil architectures (which adds moving weight and is challenging to connect electrically). The use of iron laminations for flux channeling adds weight and the use of permanent magnet limits the temperature range and ruggedness of the alternator. The use of magnet adhesives introduces potential out-gassing into the working fluid of the engine and further limits the operating temperature range often below 200 C.
Finally, the Stirling engine has fewer control options when only the stator coil is actively controlled. A doubly fed induction architecture provides additional control freedom since both the moving and stationary magnetic fields can be adjusted for both maximum efficiency and power factor. In addition, a traditional Pulse Width Modulated control system may over time, stress the permanent magnets due to higher frequency back EMF and localized heating from induced eddy currents. Moreover, the output signal may develop excessive total harmonic distortion resulting in heavy filtering requirements to reduce electromagnetic interference of sensitive on-board instrumentation.
Overall, the main disadvantage of the prior art may be summarized as limited operational temperature range (all use permanent magnets and adhesive bonding organics and hence limited to below 250° C. operation), limited efficiency (all mechanical to electrical transduction efficiencies are less than 93%), excessive weight, limited control options (all state of the art linear alternators are single-fed (active stator coil) and utilize iron laminations for flux control, all lose magnetic flux beyond the iron and require additional electromagnetic interference protection, all have eddy-current losses in iron lamination and permanent magnets, and all have a part of the cycle in which iron flux path not fully utilized due to either saturation or flux blockage. All of these disadvantages limit the applicability of Stirling technology for both space flight and aircraft applications.